1. Technical Field of the Invention
The present invention relates generally to tire inflation pressure detecting apparatuses and wireless electric power charging systems. More particularly, the invention relates to a batteryless, direct-type tire inflation pressure detecting apparatus for a vehicle which is based on a transponder method and has minimum influence on electric or electronic devices provided on the vehicle.
2. Description of the Related Art
A conventional direct-type tire inflation pressure detecting apparatus includes at least a transmitter and a receiver.
The transmitter is directly installed to a wheel of a vehicle and includes a pressure sensor working to sense inflation pressure of a tire fitted on the wheel. The transmitter is configured to transmit a pressure signal indicative of the inflation pressure of the tire sensed by the pressure sensor.
The receiver is installed to the body of the vehicle and includes an antenna. The receiver is configured to receive, through the antenna, the pressure signal transmitted by the transmitter and determine the inflation pressure of the tire based on the received pressure signal.
In the above arrangement, the transmitter is conventionally supplied with electric power by a battery provided on the wheel of the vehicle. On the other hand, developments have recently been made to supply electric power to the transmitter without using a battery.
For example, U.S. Patent Application Publication No. 2005-0280523 discloses a batteryless tire inflation, pressure detecting apparatus which is based on a transponder method.
According to the transponder method, the receiver (i.e., the interrogator) works to transmit, through the antenna thereof, a radio wave for electric power charge to the transmitter, thereby charging a charging unit of the transmitter (i.e., the transponder) with electric power. When the charging unit is completely charged with electric power, the transmitter is then activated by the charged electric power to transmit the pressure signal to the receiver.
However, in such a batteryless tire inflation pressure detecting apparatus, the radio wave for electric power charge is generally transmitted at a high strength close to the upper limit of a permissible strength range specified under regulations.
Further, such a batteryless tire inflation pressure detecting apparatus generally includes more than one transmitter, and thus the receiver transmits more than one radio wave for respectively charging the charging units of the transmitters.
Accordingly, when the radio waves for electric power charge reach and intersect in an area where automotive electric or electronic devices, such as a radio, an ECU, and a keyless entry system, are provided, they may produce strong noises that influence those devices. Particularly, when the radio waves for electric power charge are transmitted at a frequency in the LF band, the influence thereof on the automotive devices may be very large.